The present invention concerns an improved process and intermediates for the preparation of 5R,6S-6-(1R-1-hydroxyethyl)-2-(1-oxo-3-thiolanylthio)-2-penem-3-carboxylic acids, of the formula ##STR1## or a pharmaceutically-acceptable cationic salt or the pivaloyloxymethyl ester thereof, in particular a salt or ester which is an approximately 1:1 mixture of diastereoisomers in which the penem 2-substituent is (cis-1-oxo-3-thiolanylthio), i.e., the thiolane ring substituents are cis relative to each other. These compounds were first described in European patent application No. 130025, which further discloses the method of using these compounds as antibacterial agents.
The method of synthesis disclosed in EP No. 130025 is highly versatile, permitting the facile synthesis of a wide variety of analogs. However, for individual compounds, particularly the above preferred cis compound and its pivaloyloxymethyl ester, a more direct synthesis comprising fewer chemical steps is highly desirable.